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Subject:	sched_getaffinity() always returns the full cpu mask
To:	cygwin AT cygwin DOT com
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Date:	Sat, 8 Mar 2025 18:11:07 +0100
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From:	Christian Franke via Cygwin <cygwin AT cygwin DOT com>
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Testcase:

$ grep processor /proc/cpuinfo | tail -1 # i7-14700K
processor       : 27

$ sleep 60 & taskset 0x1 sleep 60 &
[1] 62094
[2] 62095

$ taskset -p 62094
pid 62094's current affinity mask: fffffff

$ taskset -p 62095
pid 62095's current affinity mask: fffffff


Another test shows that the affinity mask is correctly set to 0x1 by 
sched_setaffinity().

In sched.cc:__sched_getaffinity_sys(), GetProcessAffinityMask() is 
called, but its 'procmask' result is never used. The returned cpu_set is 
build using groupmask(__get_cpus_per_group ()) which is constant, here 
0xfffffff. The original commit 641ecb07 likely worked (until 687c4bad^) 
on older Windows versions which did not support cpu groups.

I don't know how to fix this for systems with more than one cpu group, 
so no patch provided.

It might be needed to handle interesting corner cases: "If the process 
had explicitly set the affinity of one or more of its threads outside of 
the process' primary group, the function returns zero for both affinity 
masks."
https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-getprocessaffinitymask

-- 
Regards,
Christian


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